In recent years, to reduce an amount of fuel consumed per unit travel distance (also referred to as fuel consumption), some multi-cylinder engines for motor vehicles are equipped with a cylinder deactivation mechanism for changing the number of active cylinders in accordance with an output requested by the driver (such engines may be called cylinder deactivation engines) (see Patent Document 1). In the cylinder deactivation engines, during cylinder deactivation operation, fuel supply to inactive cylinders is stopped and intake and discharge valves of the inactive cylinders are closed to reduce pumping loss, whereby fuel efficiency is improved significantly. On the other hand, in the case of a motor vehicle having a drive-by-wire engine mounted thereon, the accelerator pedal is not connected with an output control device (such as a throttle valve or a fuel injection device) by cables or the like and it is difficult to obtain a depression reaction force corresponding to an amount of depression only by use of a return spring, and therefore, an electrically driven reaction force actuator is used to provide the accelerator pedal with a depression reaction force (see Patent Document 2).